1. Field of the Invention
The invention relates to the field of cathode ray tube intensity control systems and more particularly to closed loop intensity control systems for cathode ray tubes.
2. Description of the Prior Art
Many cathode ray tube systems, including television receivers, display devices and the like have intensity controls which may be adjusted by an operator. To use such controls, the operator adjusts the intensity until it is satisfactory to him. Intensity controls of this variety control the voltage applied to the grid electrode of the electron gun and thereby control the intensity of the beam and image. Such systems provide no compensation for day to day and hour to hour variations in the control characteristics of the electron gun. Such systems are adequate for use with operator observed displays because of the wide dynamic range of the human eye.
Unfortunately such prior art systems do not provide the degree of control which is necessary in phototypesetters and other systems in which the cathode ray tube is employed to expose photoresponsive media. Such media generally have a response characteristic which is quite steep in the range which produces proper exposures. If exposed by a source whose intensity is insufficient, such media fail to produce a solid black image. On the other hand, if exposed by a source whose intensity is excessive, such media produce an image in which the transition between black and white areas of the image becomes fuzzy and the black areas become enlarged as a result of halo effects. Consequently, precise intensity control is needed in photoresponsive media exposure systems to provide high quality images which are comparable to those produced by metal type.